The present invention relates to a vacuum response type carburetor provided with an improved diaphragm device.
A vacuum response type carburetor which is mainly used for a motorcycle has a structure in which a pressure change, within a venturi passage in the carburetor, caused by opening and closing of a throttle valve is transmitted to a diaphragm disposed in a diaphragm chamber device to thereby open or close a piston valve interlocked with the diaphragm.
That is, when a throttle grip of the motorcycle is operated, a butterfly-type throttle valve provided within the venturi passage opens or closes, and then, the pressure within the venturi passage is thereby raised or lowered. This pressure change is thereby transmitted to the diaphragm chamber device, and the diaphragm disposed in the diaphragm chamber device and made of a thin rubber film is moved, so that the piston valve which is interlocked with the diaphragm is also moved to thereby adjust the passage area of the venturi passage and the amount of fuel to be delivered. Consequently, an air-fuel mixture of an optimal air-fuel ratio according to the degree of opening of the throttle valve is always provided to the engine side, thus maintaining engine properties and fuel consumption to a desired condition.
Since the diaphragm of the carburetor is perpetually exposed to a mist of gasoline which is the fuel for the engine, the diaphragm is formed, in the prior art, of a rubber material having chemical-resistant property, such as hydrine rubber or NBR (nitrile butadiene rubber).
However, qualities of gasolines are different in various countries. In some cases, the quality of the gasoline is lesser than that in Japan and, accordingly, there is a possibility of gasoline having poor quality adhering to the diaphragm, thereby advancing the deterioration of the diaphragm.
For example, since hydrine rubber is apt to swell when contacting gasoline of poor quality, a diaphragm formed of hydrine rubber may become soft and deteriorate, leading to early rupturing. On the other hand, a diaphragm formed of NBR may become hard and deteriorate, thereby obstructing smooth sliding of a piston valve assembled in a carburetor.
A high-octane gasoline and an alcohol fuel have a tendency to cause the diaphragm to deteriorate as compared to a regular gasoline.
In view of the foregoing disadvantages, in the prior art, the diaphragm is formed thicker in order to slow the deterioration from the gasoline of poor quality, the high-octane gasoline and the alcohol fuel to prevent early rupturing and obstruction of smooth sliding of the piston valve. However, when the thickness of the diaphragm is increased, the rigidity of the diaphragm increases and the flexibility thereof decreases, so that the sliding friction of the piston valve increases. As a result, the piston valve cannot slide quickly and, consequently, the throttle response is made worse.